(1) Field of the Invention
The present invention generally relates to pulse width modulation circuits, and more particularly to a pulse width modulation circuit which generates a pulse width modulated signal having a duty ratio which varies in accordance with the degree of error contained in an input signal.
(2) Description of the Related Art
A pulse width modulation circuit in which the duty ratio of a pulse width modulated signal varies in accordance with the degree of error included in an input signal is applied to, for example, a power steering control device of an automotive vehicle. The power steering control device is a device which controls the degree of force necessary to operate a steering wheel on the basis of a vehicle speed. Normally, the power steering control device operates by the pulse width modulation process. A pulse width modulated signal has a duty ratio which varies in accordance with the vehicle speed, and is supplied to a solenoid valve as an opening angle control signal. The solenoid valve is opened at an angle dependent on the duty ratio of the pulse width modulated signal, that is, the average of a driving current passing through a solenoid coil of the solenoid valve. The opening angle of the solenoid valve changes the size of a bypass oil passage of a power cylinder. When the steering wheel is turned very much or the vehicle is traveling at a low speed, the opening angle of the solenoid valve is controlled so that a light steering effort can be obtained. On the other hand, when the vehicle is traveling at a middle or high speed, the opening angle of the solenoid valve is controlled so that an appropriately heavy steering effort can be obtained.
The pulse width modulation circuit used for the above-mentioned power steering control device employs a feedback control in which the duty ratio of the pulse width modulated signal is adjusted on the basis of the difference (error degree) between the solenoid driving current and an input vehicle speed signal so that the above difference decreases. The solenoid driving current is obtained by detecting a voltage drop developed across a solenoid driving current detection resistor. However, the above-mentioned feedback control has a disadvantage in that if the detected solenoid driving current is much smaller than the real solenoid driving current due to, for example, a shortcircuiting of the solenoid driving current detection resistor, the error degree (difference) will be continuously greater than a predetermined threshold value. In such a case, the pulse width modulation circuit continuously generates the pulse width modulated signal having a maximum duty ratio (100%), so that an excessively large solenoid driving current passes through the solenoid coil.
In order to eliminate the above-mentioned problem, the feedback loop is disconnected from the pulse width modulation circuit when the error degree exceeds the predetermined threshold value, so that the duty ratio of the pulse width modulated signal is determined based on only the vehicle speed. Such a control is called an open-loop control and disclosed in, for example, Japanese Laid-Open Patent Publication No. 60-234070.
However, the pulse width modulation circuit disclosed in the above Japanese publication has a disadvantage in that the solenoid driving current increases as the power supply voltage increases because of an increase in a voltage generated by an alternator based on an increase in the vehicle speed. In order to suppress an increase in the solenoid driving current, a conventionally known pulse width modulation circuit shown in FIG. 1 is used.
Referring to FIG. 1, a vehicle speed signal from a vehicle speed sensor (not shown) is input to an error amplifier 2 via an input terminal 1. As shown in FIG. 2, the input vehicle speed signal is proportional to the vehicle speed, and equal to 3.6 V when the vehicle speed is 120 km/h. The error amplifier 2 compares the input vehicle speed signal with a signal dependent on a load current (solenoid driving current) which is input from an amplifier that will be described later. Then, the error amplifier 2 outputs an error (difference) signal to a terminal 3a of a switch circuit 3. The input vehicle speed signal applied to the input terminal 1 is directly input to a terminal 3b of the switch circuit 3. A comparator 4 compares the input vehicle speed signal with the error signal from the error amplifier 2, and outputs a select signal to the switch circuit 3. The select signal instructs the switch circuit 3 to connect the terminal 3b with a terminal 3c thereof when the error signal output by the error amplifier 2 is equal to or higher than the input vehicle speed signal applied to the input terminal 1 (in other words, when the degree of error is equal to or greater than a predetermined value). On the other hand, the select signal output by the comparator 4 instructs the switch circuit 3 to connect the terminal 3a and the terminal 3c when the error signal is lower than the input vehicle speed signal (in other words, when the error degree is less than the predetermined value).
When the error degree is less than the predetermined value, the output terminal of the error amplifier 2 is electrically coupled to the inverting input terminal of the error amplifier 2 via the terminals 3a and 3c of the switch circuit 3, a comparator 5, an inverter 7, a gate and source of an N-channel field effect transistor 8, a solenoid coil 9, a current detection resistor 10, resistors 12 and 14 and an amplifier 15. That is, a feedback loop including the above-mentioned circuit parts is established.
On the other hand, when the error degree is equal to or greater than the predetermined value, the output terminal of the error amplifier 2 is disconnected from the input of the comparator 5 due to the function of the switch circuit 3, so that the above-mentioned feedback loop is not established. In other words, an open loop is formed in which the input terminal 1 is coupled to the comparator 5, the inverter 7 and the gate of the field effect transistor 8 via the terminals 3b and 3c of the switch circuit 3.
The comparator 5 compares, at predetermined intervals, the level of a triangular wave signal generated by a triangular wave signal generator 6 with the level of the output signal of the switch circuit 3, and generates a pulse width modulated signal having a duty ratio based on the level of the output signal of the switch circuit 3. The pulse width modulated signal is inverted by the inverter 7, and then applied to the gate of the field effect transistor 8, so that the transistor 8 is turned ON or OFF.
The source of the field effect transistor 8 is grounded via the solenoid coil 9 and the current detection resistor 10 which are connected in series, and is connected to a cathode of a flywheel diode 11, an anode of which is grounded. During the time the field effect transistor 8 is ON, the solenoid driving current (load current) is allowed to pass through the solenoid coil 9. On the other hand, during the time the field effect transistor 8 is OFF, no current passes through the solenoid coil 9. In general, the load current dependent on the duty ratio of the pulse width modulated signal output by the inverter 7 passes through the solenoid coil 9.
The load current is converted into a voltage by the current detection resistor 10. The voltage developed across the current detection resistor 10 is voltage-divided by resistors 12 and 14. A voltage developed across the resistor 14 is amplified by an amplifier 15, and an amplified voltage is applied to the inverting input terminal of the error amplifier 2. As the vehicle speed increases, the level of the error signal output by the error amplifier 2 increases, so that the duty ratio of the pulse width modulated signal obtained at the output terminal of the inverter 7 increases. Thus, the ON period of the field effect transistor 8 per unit time is lengthened, and the load current passing through the solenoid coil 9 increases in response to an increase in the vehicle speed. Thereby, the opening angle of the solenoid valve increases, so that the size of the bypass oil passage of the power cylinder increases and the steering effort increases to an appropriate magnitude. When the vehicle speed does not change, the steering effort is maintained at the predetermined magnitude due to the feedback control.
On the other hand, a power supply voltage +B is divided by the resistors 13 and 14, and a divided voltage is applied to the input terminal of the amplifier 15. If the resistor 13 is not provided, the load current passing through the solenoid coil 9 during the ON period of the transistor 8 increases as the power supply voltage +B increases. Thus, it is impossible to suitably execute the power steering control. On the other hand, by using the resistor 13, it is possible to apply the divided voltage generated by the resistors 13 and 14 to the inverting input terminal of the error amplifier 2 via the amplifier 15 so that the level of the error signal decreases as the level of the output signal of the amplifier 15 increases since the error amplifier 2 subtracts the output signal of the amplifier 15 from the input vehicle speed signal. As a result, the duty ratio of the pulse width modulated signal decreases, so that an increase in the load current can be prevented. On the other hand, when the power supply voltage +B decreases, the circuit operates in a reverse way, so that a decrease in the load current can be prevented.
However, the above-mentioned conventional pulse width modulation circuit has a disadvantage as described below. When the difference between the input vehicle speed signal and the load current detection signal becomes equal to or greater than the predetermined value, the error amplifier 2 is disconnected by the switch circuit 3. At the same time, the divided voltage obtained by the resistors 13 and 14 is cut off, so that the circuit operation is affected by variations in the power supply voltage +B.
In order to eliminate the disadvantage mentioned above, an improved pulse width modulation circuit has been proposed (see Japanese Laid-Open Patent Publication No. 1-335390). Such an improved pulse width modulation circuit is shown in FIG. 3, in which those parts which are the same as those shown in FIG. 1 are given the same reference numerals. A triangular wave signal generator 20 generates a triangular wave signal having a central potential Vc based on the power supply voltage +B. As shown in FIG. 4, the triangular wave signal generated by the triangular wave signal generator 20 has a fixed period T and a fixed amplitude A.
On the other hand, the output signal of the switch circuit 3 has a voltage higher than the central potential Vc, as indicated by Vi' shown in FIG. 4(A). Thus, as shown in FIG. 4(B), the output signal of the inverter 7 has a low level while the triangular wave signal indicated by 'a' has a level equal to or higher than the output signal Vi'. On the other hand, the output signal of the inverter 7 has a high level while the triangular wave signal 'a' has a level (approximately equal to a value Vo of the power supply voltage +B obtained at this time) lower than the output signal Vi'. Thus, the output signal of the switch circuit 3 is a pulse train 'b' shown in FIG. 4(B). The pulse train 'b' having the wave height Vo has the fixed period T and a pulse width D which has been modulated by the output signal Vi' of the switch circuit 3, and corresponds to the above-mentioned pulse width modulated signal. The duty ratio now labeled Do (=D/T) increases as the level of the input vehicle speed signal increases (that is, as the vehicle speed increases) since the level of the output signal Vi' of the switch circuit 3 increases.
At this time, an average current i.sub.o defined by the following formula (1) passes through the solenoid coil 9: EQU i.sub.o =Vo.Do/R (1)
where Vo is the power supply voltage +B, Do is the duty ratio of the pulse width modulated signal, and R is the resistance of the solenoid coil 9.
When the power supply voltage +B increases from Vo to Vo(1+.alpha.) under the condition where the vehicle is traveling at a constant speed, the central potential of the triangular wave signal 'a' output by the triangular wave signal generator 20 increases in response to the increase in the power supply voltage +B, as indicated by Vc' shown in FIG. 5(A), and becomes almost the same as the output signal Vi' of the switch circuit 3. As a result, the pulse width modulated signal output by the comparator 5 is changed so that, as shown in FIG. 5(B), the wave height is equal to Vo(1+.alpha.) and the duty ratio is equal to Do(1-.beta.) (=D(1-.beta.)/T where .beta. is a duty ratio variation rate. Thus, an average current i' defined by the following formula (2) passes through the solenoid coil 9: EQU i'=Vo(1+.alpha.)Do(1-.beta.)/R (2).
According to the proposed circuit disclosed in Japanese Laid-Open Patent Publication No. 1-335390, the duty ratio variation rate .beta. is controlled so that the duty ratio decreases as the power supply voltage +B increases. Thus, the average currents i and i' become almost equal to each other. Even if the error amplifier 2 is disconnected when the error degree is equal to or greater than the predetermined value, the circuit can reduce the influence of variations in the power supply voltage +B.
FIG. 6 is a graph in which the horizontal axis represents the above-mentioned coefficient .alpha. and the power supply voltage +B, and the vertical axis represents the error degree defined by i'/i.sub.o. Characteristic curves I, II and III relate to duty ratios Do(1-.beta.) of 20%, 50% and 80%, respectively. Thus, the amount of compensation depends on the duty ratio Do(1-.beta.). Thus, a small amount of compensation is determined for a large duty ratio, and a large amount of compensation is determined for a small duty ratio.